Light emitting diode (LED) lighting technology is becoming more widely used due to having a longer lifespan, fewer hazards, and increased visual appeal compared to compact fluorescent lamp (CFL) or other types of lamps. Wide applications of LEDs for lighting, televisions, monitoring panels and/or other applications increasingly requires dimming.
There are different categories of dimming for lighting applications. In one type of dimming for lighting applications, a TRIAC dimmer circuit removes a portion of the ac input voltage to limit the amount of voltage and current supplied to lamp. This is known as phase dimming because it is often convenient to designate the position of the missing voltage in terms of a fraction of the period of the ac input voltage measured in degrees. In general, the ac input voltage is a sinusoidal waveform and the period of the ac input voltage is referred to as a full line cycle. As such, half the period of the ac input voltage is referred to as a half line cycle. An entire period has 360 degrees, and a half line cycle has 180 degrees. Typically, the phase angle is a measure of how many degrees (from a reference of zero degrees) of each half line cycle the dimmer circuit removes. As such, removal of half the ac input voltage in a half line cycle by the TRIAC dimmer circuit corresponds to a phase angle of 90 degrees. In another example, removal of a quarter of the ac input voltage in a half line cycle may correspond to a phase angle of 45 degrees.
Although phase angle dimming works well with incandescent lamps that receive the altered ac line voltage directly, it typically creates problems for LED lamps driven by a switching power converter. Conventional regulated switching power converters are typically designed to ignore distortions of the ac input voltage and deliver a constant regulated output until a low input voltage causes them to shut off. As such, conventional regulated switching power converters cannot dim LED lamps. Unless a power converter for an LED lamp is specially designed to recognize and respond to the voltage from a TRIAC dimmer circuit in a desirable way, a TRIAC dimmer can produce unacceptable results such as flickering of the LED lamp.
Another difficulty in using TRIAC dimming circuits with LED lamps comes from a characteristic of the TRIAC itself. A TRIAC is a semiconductor component that behaves as a controlled ac switch. In other words, it behaves as an open switch to an ac voltage until it receives a trigger signal at a control terminal, which causes the switch to close. The switch remains closed as long as the current through the switch is above a value referred to as the holding current. Most incandescent lamps use more than enough current from the ac power source to allow reliable and consistent operation of a TRIAC. However, the low current used by efficient power converters to drive LED lamps may not provide enough current to keep a TRIAC conducting for the expected portion of the ac line period. Therefore, conventional power converter controller designs rely on a dummy load, sometimes called a bleeder circuit, to take enough extra current from the input of the power converter to keep the TRIAC conducting.
In addition, the sharply increasing input voltage when the TRIAC fires during each half line cycle causes inrush input current ringing which may reverse several times during the half line cycle. During these current reversals, the TRIAC may prematurely turn off and cause flickering in the LED lamp. A series resistor damper may then be utilized to slow down the charging of the input capacitor, and dampen the input current ringing and prevent voltage overshoot of the input capacitor. In general, the damper circuit is external from the integrated circuit of the power converter controller and is implemented with a resistor coupled at the input of the power converter. However, use of the damper resistor and the dummy load degrades the overall efficiency of the system.
Some LED drivers use analog dimming to adjust LED brightness levels. Analog dimming adjusts brightness by changing forward current of the LEDs. For example, if an LED is at full brightness with 20 mA of forward current, then 25% brightness can be achieved by driving the LED with 5 mA of forward current. While this dimming scheme works well for lower end displays, the color of the LEDs shifts with changes in forward current, which is undesirable.
Other LED drivers use digital dimming such a pulse width modulation (PWM) to periodically switch between a determined current (e.g., logical high) and a substantially zero current (e.g., logical low) flow through the LED. This technique can adjust LED brightness while maintaining color quality. However, this technique requires a high frequency to prevent flickering that may be detectable by human eyes and/or cause digital noise, which is undesirable.
As such, a method and apparatus is desirable to overcome one or more of the aforementioned disadvantages.